From foe to friend

By Elsa YoungsteadtPhoto by Scott O'Neill

Wolbachia bacteria plague insects and other arthropods, living in their ovarian and testicular cells and dramatically affecting their reproduction. Because of their negative effects on the insects, these bacteria are considered parasites. But a study published in the May 2007 issue of the journal PLoS Biology shows how one strain of Wolbachia rapidly evolved from parasite to mutualist.

Wolbachia is transmitted only from infected mother arthropods to their offspring—so anything the bacteria can do to increase the proportion of infected females among their hosts, the better their chances of transmission. Accordingly, Wolbachia infections often change the sex ratio in insect populations in favor of females. But they can also decrease the overall reproductive success of infected insects: females with Wolbachia have fewer offspring than uninfected females. At least that’s the case early in the insect-Wolbachia relationship, but the new study shows how that can change.

Natural selection should favor Wolbachia strains that not only skew the sex ratio toward females, but that also increase female fecundity: if an infected mother insect has more offspring, the Wolbachia will have even more hosts. That’s exactly what scientists found when they kept track of a Wolbachia strain that infected the fruit fly Drosophila simulans for the first time in the 1980’s. Initially, the bacteria caused female flies to have fewer offspring. But by 2002, the Wolbachia strain had evolved so that it caused infected females to have more offspring than uninfected flies, not fewer.

This study demonstrates rapid evolution in bacteria, underscores evolutionary theory that parasites transmitted from mother to offspring should evolve toward a more mutualistic relationship with their hosts, and reveals the dynamic nature of host-parasite interactions. Evolution of mutualistic bacteria within insect cells also provides a snapshot of how eukaryotic cells may have initially acquired organelles through endosymbiosis.

2. Why does natural selection favor Wolbachia strains that cause their hosts to have female offspring but not males?

3. What change occurred in the Wolbachia strain found in California fruit flies? How long did it take for this change to evolve?

4. Would you expect the same kind of change to occur in bacteria that are transmitted from one individual to another through contact, as opposed to bacteria like Wolbachia that are inherited? Why or why not?